Trigger mechanism and sensitivity in cold-cathode tubes

Although cold-cathode trigger tubes are widely used in computer and control circuits, their firing mechanism is not completely understood, particularly when a capacitor is connected between the trigger and cathode. The paper describes an investigation of the firing mechanism with d.c. input circuits of high impedance. At breakdown of the trigger-cathode gap, relaxation oscillations occur, and it was found that the trigger current for onset of oscillations, ITosc, was related to the value of the capacitor C, according, to an approximate equation (ITosc¿IC)(C+CT)=A, where IC, CT and A are constants. The effect of the priming current and tube geometry have been examined and the results are in agreement with a simple theory which has been proposed. The effect of the trigger current on the anode breakdown has also been investigated. At onset of oscillations there is a large reduction in the anode breakdown-voltage. Thereafter, increasing the mean trigger current has only a small effect. This is particularly true of large capacitances. As a result, it is possible to describe the sensitivity in terms of the trigger capacitance needed to lower the anode breakdown voltage to a given value.